1. Field of the Invention
The invention relates to a method for bending panes in pairs, in which method the pair of panes is prebent in a horizontal position on a bending frame under the effect of the force of gravity and is then bent using a full bending mold acting on the prebent pair of panes. It also relates to a particularly appropriate device for implementing this method. Panes bent in pairs are then mainly, although not exclusively, fashioned into laminated glass windshields for vehicles.
2. Discussion of the Background
Document DE-C1-43 37 559 describes a relevant method, in which the prebent pairs of panes on a bending frame are applied from below to a solid upper bending mold. The latter is then surrounded at its edge by a skirt, which forms an annular slot with the exterior edge of the upper bending mold. Once the heated panes have been applied to the upper bending mold by means of the bending frame, air is sucked out at high speed through said annular slot. As a result, the two panes are applied by their entire surface to the upper bending mold. Together they acquire their definitive spherical configuration. The flow of air through the annular slot is also supposed to make any air inclusions between the two panes disappear.
Document EP-A2-0 531 152 discloses another relevant method. Thus, after they have been prebent in pairs on the bending frame, the panes are lifted off the latter and transferred together onto a lower press-bending mold. During bending, the panes are also bent into their definitive shapes by mechanical pressing on a solid upper mold. Such a method may lead to optical defects on the faces of the panes, because the pressing forces have necessarily to be transmitted from one pane to the other.
Document EP-B1-0 705 798 discloses a method for bending, on a frame, one or more panes superposed in pairs under the effect of the force of gravity, in which method the pane(s) deposited on a rigid prebending frame adapt, under the effect of the force of gravity, to the profile of this bending frame. In a subsequent second bending step, the panes to be bent are transferred from the external prebending frame to a moving final bending frame, situated on the inside, which can be operated purely mechanically.
The known methods are not free of disadvantages. If, for final bending, use is simply made of a bending frame by way of a lower bending mold, then it is not possible to prevent the panes from retaining, in the central region inside the edge region, a more pronounced deflection caused by the bending due to the force of gravity during the prebending phase. If, in order to avoid this disadvantage, a solid bending mold is used by way of a press-bending lower mold, then optical deformations may appear in the panes, these being due to the fact that the initial contact in the region of greatest deflection leads to deformations over a small area which cannot be completely eliminated even during the subsequent pressing operation. Furthermore, the known methods of this nature, working with mechanical presses, are, as a general rule, hampered by high build costs.
DE-A1-21 19 699 discloses a suction bending mold with a concave forming face. A suction bending mold is to be understood as meaning a device, for example in the form of a chamber, the forming face or side of which is formed by a bending contour with a solid surface. This forming face—concave in this instance—is usually provided with a number of holes, in a way known per se. Inside the chamber there are ducts and hollow spaces communicating with the holes. These may be connected, by associated suction pipes, to a vacuum generator or to a reservoir under vacuum. Operable valves in the pipes allow said ducts and hollow spaces to be placed suddenly under vacuum. In consequence, the air is sucked through the holes from the outer face of the suction bending mold giving rise, at points, to very large pressure differences and flow rates. According to the aforementioned document, a pane prebent on a solid upper suction bending mold is allowed to drop onto the concave lower suction bending mold. Its edges are the first to come into contact with the lower mold. The pane sinks down onto it, on the one hand, under the effect of the force of gravity, but, on the other hand, is also pressed against the forming face by the pressure difference between atmospheric pressure and the depression created in the lower mold.
This method is admittedly recommended for the simultaneous bending of two or more panes, even for high curvatures, but the transfer of a pair of panes from the convex upper mold to the concave lower mold is not, however, described. Furthermore, in this method, the two main faces of the panes necessarily come into contact with the faces of the molds.
Document U.S. Pat. No. 4,894,080 describes another method for bending pairs of panes which are prebent under the effect of the force of gravity on a prebending mold in the form of a frame, possibly produced in divided and movable form. A solid bending mold with a concave forming face is raised from beneath by the prebending mold in the form of a frame, after the latter has been positioned, and takes up the pair of panes. Using another bending mold in the form of a box, with a frame-shaped forming face, the edge of the pair of panes is pressed against the concave forming face, while inside the box, a raised pressure is created, under the effect of which raised pressure the pair of panes is pressed flat against the concave forming face. After a certain length of time, the raised pressure is raised again. The pair of panes is once again placed on the prebending mold in the form of a frame and, in place thereon, is removed from the bending station so that it can be cooled.
EP-B1-0 530 211 describes a device for bending individual panes, which device comprises a solid concave lower (suction) bending mold, onto which panes heated to the softening temperature are deposited. An upper bending frame with a complementary convex surface is then lowered onto the edge of the pane, to press the edges thereof onto the edge of the lower bending mold. Finally, the air between the lower bending mold and the pane is sucked out by applying pulses of vacuum to the forming chamber, the load applied to the edge by the bending frame ensuring sealing against the outside. The pane is brought into contact over its entire surface with the lower bending mold and thus acquires its definitive spherical curved configuration. In the last document cited, there is no question of bending panes in pairs.
According to another known method (DE-C1-197 25 189) for bending panes with the transfer of individual panes between different suction bending and prebending molds and a transfer device makes use of a bending frame split into segments. These segments can be parted from one another to allow the passage from a conveyor of a solid convex (suction) bending mold intended to receive the pane which has not yet been bent. Once the pane has been lifted up off the conveyor using the bending mold, the segments are once again brought together into a closed forming frame which, by way of a lower press-bending mold, presses the edges of the pane against the solid mold.
One advantage of concave suction concave molds is that the pane is bent in the face of the pane by suction without mechanical contact with the mold. As a result of this, imprints of the fabric of the mold into the surface of the glass are, to a large extent, avoided. This has a positive influence on the optical properties (transmission).
Finally, document EP-B1-448 447 discloses a method for bending individual panes or several panes simultaneously, in which method the (lowermost) pane is first of all placed along a first peripheral line on a first preforming mold in the form of a frame, and sinks down into a prebending contour—transverse bending—under the effect of the force of gravity. Next, the lowermost pane is held along a second peripheral bending line, which takes the place of the first peripheral line, to create the final bending contour—longitudinal bending. For this, a second bending mold in the form of a frame is placed in place of the prebending mold, the latter, depending on the embodiment, coming into contact only with a partial periphery of the surface of the glass. In this case too, the panes are bent merely under the effect of the force of gravity, the transfer from the prebending mold to the second bending mold taking place relatively swiftly. The two peripheral lines along which the face of the supported lowermost pane comes into contact with the two bending molds acting in turn, are different, because the support faces acting in turn are nested one inside the other, or respectively run parallel to one another in perpendicular projection.